Why is the preaching of the Cross so offensive?

It's the PREACHING thing that gets irritating. If you want to believe that some some human son of some mythical being was tortured and then nailed to a cross, that's your choice. But if your belief in God/Allah/Jesus/Thor/Zoroaster causes you to try to take away the rights of others because they don't believe as you do, then don't expect a warm reception.